This invention relates to feedback control systems, specifically to systems which are automatically adjusted to match performance criteria. This invention is suited for use in various applications, such as acoustic active noise control systems.
Feedback is often used to improve the performance of a system to be controlled (also known as a plant). Feedback improves the performance of an open-loop system by reducing the effects of disturbance noises and by reducing the sensitivity of the system to changes in the open-loop transfer function response. Thus, the output of the plant is able to track the input reference signal more accurately. Feedback control systems, also known as closed-loop control systems, are ubiquitous in modern day technology. Some applications include: automobile cruise control, missile guidance, chemical process control, robotic control, and active noise control. In active noise control devices, undesirable acoustic noise at the system output is reduced by producing a signal, through feedback, to destructively interfere with the noise.
The performance of a feedback system is determined by its open-loop transfer function, which includes the plant. The spectra of the noise attenuated by a stable feedback system is related to the frequency region were the magnitude of the open-loop transfer function is greater than unity (often called the control bandwidth). As the frequency span of the control bandwidth increases the frequency span of noise reduction increases until instability is approached. As the amplitude of the open-loop transfer function is increased, in the control bandwidth, the performance of noise attenuation increases until instability is approached. Although feedback control systems exhibit a reduced sensitivity to changes in the plant, compared to open-loop systems, the performance of a feedback system will change as the plant characteristics change. Therefore, the basic feedback system will suffer performance degradation if the plant is not stable.
Unfortunately, some plants have widely varying characteristics. An example of this is an active control headset disclosed in U.S. Pat. No. 5,182,774. This type of noise-attenuating headset employs a basic feedback system with a microphone and a speaker. The sensitivity of the microphone and speaker may change significantly as temperature and humidity conditions change. Also, the acoustic response of the earcup cavity in the headphones varies due to acoustic leaks around the earcusion and differences in ear geometries. In order to avoid instability, the open-loop gain and control bandwidth are reduced to account for these changes in the plant. This results in sacrificed performance.
Some feedback controllers, often called xe2x80x9cself-tuningxe2x80x9d, measure parameters of the closed-loop system during operation, and use this information to modify these parameters in a compensation filter. These systems are typically used in proportional integral differential (PID) controllers as seen in U.S. Pat. No. 5,159,547. The proportional, integral, and derivative gain constants are measured in the closed-loop system, and these parameters are then updated. But many controllers, such as those used for active noise control systems, use a more sophisticated compensation filter which uses many more than three parameters to define the filter as in U.S. Pat. No. 4,455,675. These more sophisticated compensation filters have many poles and zeros, and are defined by a transfer function curve. It would be very difficult to use a self-tuning controller in this case, because of the complex transfer function.
Another related technology uses an open-loop system and a model reference as seen in U.S. Pat. No. 5,386,477. This system uses a digital filter as a model reference, and adapts filters so that the overall feedforward response matches that of the model reference. Feedforward controllers do not perform as well as feedback controllers for many applications, especially when attenuating non-repetitive noise at the output. The feedforward model reference system will not work in a feedback system.
The inventor has discovered that a feedback system that maintains a constant open-loop transfer function, even when the plant or other components within the feedback loop change with time, would allow one to maximize the performance of the overall system.
The invention described herein is a simple solution to problems stated above. Without breaking the control loop, an embodiment of the invention measures the residual-loop transfer function during operation, and compares the estimated open-loop transfer function to a reference open-loop transfer function. The reference open-loop transfer function is determined by the designer to yield optimal performance of a given plant. A digital compensation filter in the controller adapts so that the actual open-loop transfer function sufficiently matches the reference. This results in a simple and inexpensive yet robust feedback controller that maintains a relatively constant open-loop transfer function even when components of the feedback system change over time. In this way the performance of the plant is optimized.